With the progress of small-size, high-speed and lightweight electronic devices such as cellular phones and personal computers, the capacitor used for these electronic devices is demanded to have a smaller size, a larger capacitance and a lower ESR.
An electrolytic capacitor composed of a conductor (anode body) as one electrode, a dielectric layer formed on the surface of the electrode, and a counter electrode provided thereon (semiconductor layer).
As an example of such a capacitor, the electrolytic capacitor has been proposed, which capacitor is produced by anodically oxidizing an anode body for capacitors comprising a sintered body made of valve-acting metal powder such as tantalum which can be anodized to form a dielectric layer made of the oxide of the metal in the surface of the anode body.
The electrolytic capacitor using tungsten as a valve-acting metal and employing the sintered body of the tungsten powder as an anode body can attain a larger capacitance compared to the electrolytic capacitor obtained at the same formation voltage by employing the anode body of the same volume using the tantalum powder having the same particle diameter. However, the electrolytic capacitor having the sintered body of the tungsten powder has been unpracticed as an electrolytic capacitor due to the large leakage current (LC). In order to solve this issue, a capacitor using the alloy of tungsten and other metals has been studied and has achieved some improvement in the leakage current, but it was not enough (JP-A-2004-349658 (U.S. Pat. No. 6,876,083); Patent Document 1).
Patent Document 2 (JP-A-2003-272959) discloses a capacitor using an electrode of a tungsten foil selected from WO3, W2N and WN2 having formed thereon a dielectric layer, but the capacitor is not to solve the above-mentioned leakage current problem.
Also, Patent Document 3 (WO 2004/055843 publication (U.S. Pat. No. 7,154,743)) discloses an electrolytic capacitor using an anode selected from tantalum, niobium, titanium and tungsten, but it does not describe a specific example using tungsten in the specification.
A semiconductor layer as a counter electrode of the electrolytic capacitor having a valve-acting metal sintered body as an anode can generally be composed of an inorganic semiconductor such as manganese dioxide and an organic semiconductor such as a conductive polymer to which a dopant is doped. In order to obtain particularly low ESR, a method of fabricating a semiconductor layer has been implemented by performing polymerization on an anode body having a dielectric layer to form a conductive polymer layer. The above polymerization is conducted by methods such as a chemical polymerization method, an electrolytic polymerization method in which electricity is supplied by an external electrode, an electrolytic polymerization method by way of applying electric current to an anode body, or combination thereof.
Among these, in the electrolytic polymerization method, it has been attempted to form a highly-reproducible and stable conductive polymer layer by electrolytic polymerization in which applied electric current is limited in each anode body having a dielectric layer formed thereon.